The non-aqueous secondary battery including a lithium ion secondary battery has big characteristics in light weight, high energy and long life time, and thus has been widely used as a power source of various mobile electronic devices. In recent years, its application has been expanding to industrial use represented by a power tool, such as an electromotive tool, etc.; vehicle use, such as electric vehicles, electromotive bicycles, etc. It has been noticed further in a power storage field, such as a housing battery system, etc.
As an electrolyte solution for the lithium ion secondary battery operable at normal temperature, use of the non-aqueous electrolyte solution is desirable in view of practical use. A combination of, for example, a highly dielectric solvent, such as a cyclic carbonate ester, and a low viscosity solvent, such as a lower linear carbonate ester, is exemplified as a general solvent. However, a normal highly-dielectric solvent has a high melting point, as well, as may cause deterioration of load characteristics (rate characteristics) and low-temperature characteristics of the non-aqueous electrolyte solution depending on type of an electrolyte salt used in the non-aqueous electrolyte solution.
As one type of a solvent to overcome such a problem, nitrile-based solvents superior in balance between viscosity and dielectric constant have been proposed. Among them, acetonitrile has high potential as a solvent used in the electrolyte solution of the lithium ion secondary battery. However, acetonitrile has fatal defect of being reductively and electrochemically decomposed at a negative electrode, therefore exertion of practical performance has not been attained. Several improvement ideas have been proposed against this problem.
Main improvement ideas that have been proposed up to now are classified to the following three.
(1) Method for Protecting Negative Electrode and Suppressing Reductive Decomposition of Acetonitrile by Combination of Specific Electrolyte Salt and Additives, etc.
For example, in PATENT LITERATURE 1 and 2, there has been reported the electrolyte solution where influence of reductive decomposition of acetonitrile is reduced by combination of acetonitrile, as a solvent, with a specific electrolyte salt and additives. In addition, at the dawn of the lithium ion secondary battery, there has also been reported the electrolyte solution containing a solvent obtainable by only diluting acetonitrile with propylene carbonate and ethylene carbonate, as in PATENT LITERATURE 3. However, in PATENT LITERATURE 3, high-temperature durability performance was judged only by evaluation of internal resistance and battery thickness after high-temperature storage, therefore information on whether it practically operates as a battery when it is placed under high-temperature environment has not been disclosed. It is very difficult to suppress reductive decomposition of the electrolyte solution containing an acetonitrile-based solvent, by measures of simple dilution only with ethylene carbonate and propylene carbonate. As a suppression method for reductive decomposition of a solvent, a method for combining a plurality of electrolyte salts and additives is practical, as in PATENT LITERATURE 1 and 2.
(2) Method for Suppressing Reductive Decomposition of Acetonitrile by Using Negative Electrode Active Material Which Occludes Lithium Ions at Higher Potential than Reductive Potential of Acetonitrile.
For example, in PATENT LITERATURE 4, there has been reported that a battery, which avoids reductive decomposition of acetonitrile, can be obtained by using a specific metal compound in a negative electrode. However, in applications putting importance on energy density of the lithium ion secondary battery, a method for using the negative electrode active material which occludes lithium ions at lower potential than reduction potential of acetonitrile is far more advantageous, in view of potential difference. Accordingly, when the improvement ideas of PATENT LITERATURE 4 are used in such applications, they are disadvantageous because of providing a narrow usable voltage range.
(3) Method for Maintaining Stable Liquid State by Dissolving High Concentration of Electrolyte Salt in Acetonitrile.
For example, in PATENT LITERATURE 5, there has been described that reversible reaction between lithium insertion to a graphite electrode and lithium desorption from a graphite electrode is possible, by using an electrolyte solution with 4.2 mol/L of lithium, bis(trifluoromethanesulfonyl)imide (LiN(SO2CF3)2) dissolved in acetonitrile. In addition, in PATENT LITERATURE 6, there has been reported that a reaction between Li+ insertion to graphite and Li+ desorption from graphite was observed, and further high-rate discharge was possible, as a result of charge-discharge measurement on a cell using the electrolyte solution with 4.5 mol/L of lithium bis(fluorosulfonyl)imide (LiN(SO2F)2) dissolved in acetonitrile.